This invention relates to a zinc oxide (ZnO) varistor material and a method of producing same.
It is widely known that a sintered ZnO mixed with small amounts of bismuth oxide (Bi.sub.2 O.sub.3) and other additives has high non-linear current-voltage characteristics. Such a material, generally called varistor material, has been widely applied to the voltage stabilization or to the absorption of transient surge in electric circuits by taking advantage of the non-linear between its voltage and current. The relationship between the electric current and voltage of a varistor may be expressed by the following empirical equation: EQU I=(V/C).sup..alpha.
wherein V represents an electric voltage applied to the varistor, I represents an electric current passing therethrough, C is a constant and .alpha. is a non-linear coefficient. The non-linear coefficient .alpha. is calculated according to the following equation: EQU .alpha.=log (I.sub.2 /I.sub.1)/log(V.sub.2 /V.sub.1)
wherein V.sub.1 and V.sub.2 each represent the electric voltage at given current I.sub.1 and I.sub.2.
I.sub.1 and I.sub.2 are generally determined at 1 mA and 10 mA, respectively and V.sub.1 is called a varistor voltage. The non-linear coefficient varies with the composition and production method of the varistor material. Generally speaking, a varistor material with as large a non-linear coefficient as possible is preferred.
Although several theories have been reported relating to the mechanisms of the expression of non-linear current-voltage characteristics of ZnO varistors, no definite one has been established so far. However, it is recognized that the electric properties of a varistor originate from its grain boundaries. A ZnO varistor generally contains ZnO grains around which a highly resistant boundary layer is located and bound thereto. Additives are employed in order to form this boundary layer. A number of additives are generally used and the types and amounts thereof may vary depending on the properties sought.
A ZnO varistor material has been hitherto prepared as follows. Several additives such as oxides of Bi, Co, Mn, Sb, Cr and the like metals are mixed with ZnO powder and dried. The dried mixture is molded into a desired shape and subsequently sintered. During the sintering stage, the mixture is reacted to give a varistor material. A varistor element is obtained by fitting electrodes and conductors to the varistor material.
Known ZnO varistor materials have a varistor voltage of about 200 V/mm. Thus, when a high varistor voltage is desired, such as in the case of utilization in a lightning arrester, such varistors must have a large thickness. For example, a thickness of about 3.5 m is required for obtaining a varistor voltage of 700 KV with a varistor material having a varistor voltage of 200 V/mm. Such a large varistor element causes a difficulty in electrical insulation, a large increase in production costs and a limitation in selecting the installation position. Thus, there is a great demand for a varistor material with a high varistor voltage.
It is known that the voltage drop per grain boundary of a ZnO varistor is about 2-4 V and is independent from the composition or production process parameters. Therefore, if the growth of grains at the sintering stage can be suppressed, a varistor material with a high varistor voltage per unit thickness may be obtained.
However, ZnO varistor materials generally contain bismuth oxide, strontium oxide or barium oxide which forms a liquid phase on the boundary layers at the sintering stage to accelerate the growth of grains. For the purpose of suppressing the growth of grains in such ZnO varistors, the following methods are proposed. One proposal is to effect the sintering at a low temperature of up to 1100.degree. C. Since sintering fails to proceed effectively at such a temperature, however, it is necessary to adopt a special measure. As a result, the production method becomes complicated and is difficult to perform quality control. Another proposal is to use an inhibitor such as antimony oxide or silicon oxide. Since such an inhibitor should be used in a relatively large amount in order to obtain a desired result, problems are caused with respect to heterogeneity of the product and reduction of surge resistance.
A varistor material containing ZnO and ZnMn.sub.2 O.sub.4 is proposed in U.S. Pat. No. 5,073,303 and in U.S. Pat. No. 5,076,797. No specific examples are disclosed in this prior art which show varistors with a varistor voltage of 800 V or more per 1 mm of the thickness thereof. Further, it is described that the desired high non-linear coefficient cannot be obtained when the content of MnO is outside of a range of 3-7 mole % based on a total of ZnO and MnO.